I know the thread is older but with the release of the new Shearwater, I was also thinking why doesn't anyone offer mail order type rental? Maybe I will buy one and start renting it. I noticed someone mentioned the liability risk. How is it a liabilty risk to rent a computer to a stranger, but is perfectly OK to sell one to some random stranger on the board with no worries about liability?
I suspect that there is some legal mumbo jumbo that relates to "fit for purpose" and a difference between selling something "as-is" and renting something.
I would guess that, if a private individual sells a computer, they can claim that they did not assert any fitness for purpose. I.e. you sold it "as-is" and did not claim that it is fit for use as a dive computer. Many private sellers specifically mention "you should have this serviced" before you use it - for any scuba gear. That could be argued as passing the liability for fitness of purpose to the buyer and whoever they get to service it.
On the other hand, if you're renting out a dive computer, it would be a lot harder to argue that you were not offering as fit for the specific purpose of diving with it. So, if it tells them they're at 30' when they're really at 130', the liability is more easily pinned on you.
The simple answer is to form an LLC (or LLP or C corp or whatever) and have the LLC own the computers, rent them out, and, thus, "own" the liability. Having that structure won't prevent an owner of the LLC from being liable for gross negligence. But, as long as the company doesn't do anything grossly negligent, liability should stop there and worst case is you lose all your computers....
That said, the real concern is not losing a lawsuit. The real concern is getting sued at all. Even if you have a slamdunk case that you're not liable, the cost to defend yourself in a lawsuit could break you - if you don't have decent liability insurance. And the cost of liability insurance to rent out any kind of scuba equipment probably makes it financially impractical to run a "casual" side business of renting out your computers.